This invention relates generally to methods and apparatus for drying objects and, more particularly, to such methods and apparatus for drying an object constituted by a porous web-like material, such as a paper web, a granular material such as peat, or a solid material, such as wood.
Preliminarily, the method and apparatus of the present invention will be described below mainly in connection with an application whereby a paper web is dried. However, it is understood that the method and apparatus of the present invention are equally applicable in connection with drying granular material, such as peat, and solid material, such as wood. In this connection, examples are set forth below whereby the present invention is applied to the drying of timber and of peat. Among the various applications of the drying apparatus of the method of the invention are, among others, the drying of various textile webs, leather, various types of sheet and board products, other types of web-like products, granular and powdery products such as chemicals, fodders, peat and the like.
It should also be noted that the method and apparatus of the present invention are described below in connection with the removal of water from the object to be dried since such dewatering constitutes the most important application of the present invention. However, it is understood that the present invention is equally applicable to the removal of other liquids from an object to be dried.
Conventionally, a porous paper web running through a paper machine is dried initially by dewatering on a fabric, such as a wire, or between two fabrics. Such initial dewatering reduces the moisture content of the paper web to a value u.sub.v =5.7 to 2.3 (g of H.sub.2 O per g of dry matter), depending upon the brand of paper. Subsequently, further removal of water from the web is accomplished in the press section of the paper machine by passing the web in the nips of press rolls in which a porous felt is generally also applied to enhance the dewatering. The moisture content of the paper web is generally reduced in the press section of the paper machine to a value u.sub.v =1.6 to 1.2. Following the press section, the paper web is dried through evaporation, e.g., utilizing multiple cylinder dryers, where the web to be dried is placed in contact with steam-heated, smooth-surfaced drying cylinders. The ultimate moisture content of the paper web is generally in the range u.sub.v =0.05 to 0.1.
The above-described method of drying a paper web is not energy efficient. Thus, it need only be noted that drying by evaporation consumes remarkable quantities of energy since the energy required for evaporation of water is about 2500 kJ/kg.